Highlights

As the Syrian conflict enters its seventh year, more than 465,000 Syrians have been killed in the fighting, more than a million injured and over 12 million Syrians - half the country's prewar population - have been displaced from their homes.

In 2011, what became known as the Arab Spring revolts toppled Tunisian President Zine El Abidine Ben Ali and Egyptian President Hosni Mubarak.

That March, peaceful protests erupted in Syria as well, after 15 boys were detained and tortured for having written graffiti in support of the Arab Spring. One of the boys, 13-year-old Hamza al-Khateeb, was killed after having been brutally tortured.

The Syrian government, led by President Bashar al-Assad, responded to the protests by killing hundreds of demonstrators and imprisoning many more. In July 2011, defectors from the military announced the formation of the Free Syrian Army, a rebel group aiming to overthrow the government, and Syria began to slide into civil war.

What caused the uprising?

Initially, lack of freedoms and economic woes fuelled resentment of the Syrian government, and public anger was inflamed by the harsh crackdown on protesters. Successful uprisings in Tunisia and Egypt energised and gave hope to Syrian pro-democracy activists. Many Islamist movements were also strongly opposed to the Assads' rule.

In 1982, Bashar's father, Hafez al-Assad, ordered a military crackdown on the Muslim Brotherhood in Hama, which killed between 10,000-40,000 people and flattened much of the city.

Although the initial protests in 2011 were mostly non-sectarian, armed conflict led to the emergence of starker sectarian divisions. Minority religious groups tend to support the Assad government, while the overwhelming majority of opposition fighters are Sunni Muslims.

Most Syrians are Sunni Muslims, but Syria's security establishment has long been dominated by members of the Alawite sect, of which Assad is a member.

The sectarian split is reflected among regional actors' stances as well.

Even global warming has been claimed to have played a role in sparking the 2011 uprising. A severe drought plagued Syria from 2007-10, spurring as many as 1.5 million people to migrate from the countryside into cities, which exacerbated poverty and social unrest.

International involvement

Foreign backing and open intervention have played a large role in Syria's civil war. An international coalition led by the United States has bombed targets of the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group since 2014.

The US has repeatedly stated its opposition to the Assad government, but has hesitated to involve itself deeply in the conflict, even after the Assad government allegedly used chemical weapons in 2013, which former US President Barack Obama had referred to as a "red line" that would prompt intervention.

In October 2015, the US scrapped its controversial programme to train Syrian rebels, after it was revealed that it had spent $500m but only trained 60 fighters.

The CIA froze funding and logistical support in February 2017, and then announced the programme's shut down ahead of Trump's meetings with Putin at the G20summit.

On April 7, the US carried its first direct military action against Assad's forces, launching 59 Tomahawk cruise missiles at a Syrian air force base from which US officials believe a chemical attack in Khan Sheikhoun had been launched.

Russia launched in September 2015 a bombing campaign against what it referred to as "terrorist groups" in Syria, which included ISIL as well as rebel groups backed by western states. Russia has also deployed military advisers to shore up Assad's defences.

At the UN Security Council, Russia has vetoed eight Western-backed resolutions on Syria, while China vetoed six resolutions.

On May 4, Russia, Iran and Turkey called for the setup of four "de-escalation zones" in Syria, in which Syrian and Russian fighter jets are not expected to fly over for six months.

However, as of July 6, the three countries failed to agree on the details of the ceasefire agreement, such as the policing of the four safe zones and their boundaries.

Several Arab states, along with Turkey, have provided weapons to rebel groups in Syria. The governments of majority-Shia Iran and Iraq support Assad, as does Lebanon-based Hezbollah, while Sunni-majority states including Turkey, Qatar, Saudi Arabia and others staunchly support the rebels.

An investigativereportpublished late August alleges that Saudi Arabia , the UAE and US military used Azerbaijani airlines to transport large quantities of weapons, which ended up in the hands ISIL and Kurdish fighters, among other armed groups.

Turkish troops and special forces backed by the Free Syria Army, launched in August 2016 operation "Euphrates Shield" against ISIL to liberate the strategic Syrian city of Jarablus on the border with Turkey and to stop the advance of Kurdish militia fighters. Turkey's government fears its large native Kurdish population may grow more restive and demand greater autonomy as a result of increased Kurdish control in northeast Syria.

In March 2017, Turkey officially ended the Euphrates Shield military operation, but struck again in April against Kurdish PKK targets in the Karachok Mountains.

Turkey's top officials have also criticised the US' decision to arm Kurdish fighters battling ISIL in Syria.

Israel also carried out air strikes inside Syria, the latest of which on Damascus and Quneitra. Israel and Syria are technically at war since 1948, but the border remained largely quiet since 1973.

Rebel groups

Since the Free Syrian Army formed in 2011, many new rebel groups have joined the fighting in Syria, including ISIL, Jabhat Fateh al-Sham, Iran-backed Hezbollah, and the Syrian Democratic Forces (SDF) dominated by the Kurdish People's Protection Units (YPG).

The FSA has weakened as the war has progressed, while explicitly Islamist groups such as the al-Nusra Front became empowered. al-Nusra Front leader, Abu Mohammed al-Joulani, announced in 2016 his group's name changed to Jabhat Fateh al-Sham, or The Front for liberation of al-Sham, and severed ties with al-Qaeda.

ISIL emerged in northern and eastern Syria in 2013 after overrunning large portions of Iraq. The group quickly gained international notoriety for its brutal executions and its energetic use of social media. The ranks of ISIL include a sizeable number of fighters from around the world.

Kurdish groups in northern Syria are also seeking self-rule in areas under their control.

Lebanese members of Hezbollah are fighting on the side of Assad, as are Iranian and Afghan fighters.

In December 2016, the Syrian army announced that Aleppo has been fully recaptured from rebel fighters, the government's biggest victory in the nearly six-year civil war.

Syrian government forces used chemical weapons in rebel-held areas of Aleppo during the final weeks of the battle to retake the key city, killing at least nine people and wounding hundreds more, according to Human Rights Watch.

Since Assad's forces recaptured Aleppo, a new military alliance of rebel groups in northern Syria was formed with the aim to consolidate military control over Idlib province, the western part of Aleppo province and parts of Latakia province, according to an FSA commander.

Whereas several rounds of peace talks have failed to stop the fighting, the Syrian government and the opposition groups have agreed to 13 evacuations. Evacuation deals allow opposition fighters to safely leave government-besieged cities and towns for areas under opposition control in Northern Syria.

Rebel groups have jockeyed for power, and frequently fight one another. Fighting has occasionally spilled over from Syria into Lebanon, contributing to the country's political polarisation.

The situation today

Syrian troops entered Deir Az Zor from the east on September 18, increasing chances of a standoff with US-backed SDF forces operating nearby. Earlier in September, there had been an ISIL evacuation from Lebanon to Deir Az Zor.

Syrian refugees

The Syrian war is creating profound effects far beyond the country's borders. Lebanon, Turkey, and Jordan are now housing large and growing numbers of Syrian refugees, many of whom have attempted to journey onwards to Europe in search of better conditions.

The UN recently reported that around 440,000 displaced Syrians returned to their homes in the past year. According to UNHCR, those displaced mainly returned to Aleppo, Hama, Homs and Damascus to find family members and check on their property.

Human Rights Watch also reported that governments and aid agencies have been unable to account for millions of dollars in school aid for Syrian refugees. This led to severe funding gaps that adversely affected 50,000 children.

With much of Syria in ruins, millions of Syrians having fled abroad, and a population deeply traumatised by war, one thing is certain: Rebuilding Syria after the war ends will be a lengthy, extremely difficult process.